1. Field of the Invention
The present invention relates to a magnetic video recording apparatus and particularly to a magnetic video recording apparatus in which a magnetic tape is made to travel intermittently for recording.
2. Description of the Prior Art
In a conventional video tape recorder currently on the market, for example, a video tape recorder of the VHS system, the tape travelling speed is 33.35 mm/sec.+-.0.5% in the NTSC standard mode, and if a video tape of the type of T-120 is used, the recording time is approximately two hours. In order to extend the recording time, a method is adopted in which the tape travelling speed is reduced to 1/3 so as to make recording for approximately six hours using the tape of the type of T-120. However, as a video tape recorder for surveillance in the banks, parking lots, etc., the recording time is still insufficient.
Therefore, in order to make recording for a further long period of time, a magnetic video recording apparatus for intermittent recording in which a magnetic tape is made to travel intermittently is used these days for surveillance in the bank etc. However, in such a conventional magnetic video recording apparatus for intermittent recording, a magnetic tape travels intermittently and recording is made during a period when the magnetic tape is stopped, and accordingly the tape pattern of the magnetic tape recorded by such apparatus does not conform with the standard tape format of an ordinary video tape recorder currently on the market, in which a tape is made to travel at a fixed speed for continuous recording. Accordingly, in order to reproduce a magnetic tape recorded by a magnetic video recording apparatus for intermittent recording, a special reproducing apparatus other than an ordinary video tape recorder is needed, although a magnetic tape of the same type as in an ordinary video tape recorder is used. This will be explained in detail using drawings.
FIG. 1 is a flow chart showing the recording operation in a conventional magnetic video recording apparatus for intermittent recording. In the step S1, recording start operation is made and then, in the step S2, a magnetic tape is loaded. More specifically, a magnetic tape is withdrawn from the cassette and wound on a rotating drum. In the step S3, a video signal is recorded in the magnetic tape, for one frame for example, in a state where the tape is stopped. In the step S4, the magnetic tape is made to travel and is stopped in a position for recording the next one frame. This stop state of the tape continues till the next recording timing. Then, at the next recording timing, recording is made for one frame (in the step S5), and in the step S6, the magnetic tape is newly made to travel for one frame and is stopped. The above described operations are repeated so that recording is made intermittently.
If a magnetic tape in which recording was made intermittently in the stop periods of the magnetic tape as described above is reproduced by an ordinary video tape recorder which makes recording and reproducing continuously at a fixed speed, inconveniences as described below will occur because of disaccord of the tape formats. In case where the azimuths of the adjacent video tracks are different, the demodulated picture is considerably influenced by noise because of a lack of FM waves due to the azimuth error. On the other hand, in case where the azimuths of the adjacent video tracks are the same, two FM waves simultaneously enter in a reproducing system or insufficiency occurs in a reproduction output, which causes troubles in demodulation. Inconveniences caused by the disaccord of the tape formats as described above will be described in more detail in the following by taking an example of a video tape recorder of the VHS system.
FIGS. 2A and 2C show a relation between a tape pattern recorded by a conventional magnetic video recording apparatus for intermittent recording and a locus of a reproducing head in a video tape recorder of the VHS system. FIGS. 2B and 2D respectively show waveforms of reproduction outputs in the cases of FIGS. 2A and 2C. FIGS. 2A and 2C differ only in the tracking start points of the reproducing heads. The tracking start point can be adjusted in an ordinary video tape recorder. Referring to FIGS. 2A and 2C, in a magnetic tape 1 recorded by a conventional magnetic video recording apparatus for intermittent recording, a tape pattern P.sub.1 as shown in drawings is formed. More specifically, in the magnetic tape 1, video tracks T.sub.A and T.sub.B are formed alternately by means of two magnetic heads provided in a rotating drum and having different azimuth angles. In this case, recording is made during the stop of the magnetic tape 1 and, therefore, the angle between the video tracks T.sub.A and T.sub.B and the tape travelling direction is small. On the other hand, in case where the magnetic tape thus recorded is reproduced by an ordinary video tape recorder of the VHS system, the angle of the locus P.sub.0 of a reproducing head with respect to the tape travelling direction becomes large since reproduction is made while the magnetic tape travels. More specifically, since the angle of the video tracks T.sub.A and T.sub.B and the angle of the locus P.sub.0 of the reproducing head are different, the waveform V of the reproduction output from the reproducing head considerably changes according to a lapse of time as shown in FIGS. 2B and 2D, and insufficiency occurs in the reproduction output. Referring to FIGS. 2B and 2D, the waveforms V.sub.A and V.sub.B of the reproduction outputs are respectively waveforms of the outputs from the reproducing heads having the same azimuth angle as that of the recording heads which record the video tracks T.sub.A and T.sub.B. Insufficiency in the reproduction output causes noise in the reproduced picture and accordingly the reproduced picture becomes extremely indistinct. Comparing the tape pattern in FIG. 2A and the tape pattern in FIG. 2C, a larger output of reproduction can be obtained from the tape pattern in FIG. 2C. With this tape pattern, however, still a large noise portion appears in the reproduced picture.
FIG. 3A shows relation between a tape pattern recorded by a conventional magnetic video recording apparatus for intermittent recording having a plurality of recording heads of the same azimuth angle and a locus of a reproducing head in a video tape recorder of the VHS system. FIG. 3B shows reproduction output waveforms in the case of FIG. 3A. The tape pattern P.sub.2 in this case is composed of video tracks T.sub.1, T.sub.2, . . . all having the same azimuth angle. On the other hand, the reproduction output waveforms V are those as shown in FIG. 3B, since in a video tape recorder of the VHS system, two magnetic heads have different azimuth angles. Specifically stated, in the first field, a reproduction output waveform V.sub.12 (or V.sub.34) is obtained, but in the second field, no reproduction output is obtained and as a result a noise appears over the whole surface of the picture to be reproduced.
Although the foregoing description was made of a case using a video tape recorder of the VHS system for reproduction, it is just the same with a case using a video tape recorder of the .beta. system or the CVC system or a video tape recorder for business use and a reproduced picture without noise or with little noise cannot be obtained either in such a case. Since video tape recorders of the VHS system, the .beta. system etc. are widely utilized at present, it is extremely inconvenient that a magnetic tape intermittently recorded cannot be reproduced by such an ordinary video tape recorder put on the market.